Oxygen sensors are used in motor vehicles to measure oxygen in the vehicle exhaust. Measurements from the sensors aid in adjusting the operating parameters of the vehicle, and especially in adjusting the operating parameters to reduce hydrocarbon and other emissions and to improve fuel economy.
Oxygen sensors are typically mounted in the exhaust manifold and/or just after the catalytic converter. In either location the sensors are exposed to water vapor included in the exhaust gases. Oxygen sensors are designed to operate at an elevated temperature, usually with the sensing element of the sensor heated to a temperature greater than about 600° C. To achieve the elevated temperature, the oxygen sensors include a heater rod. A problem exists if a sensor is rapidly heated by the heater rod to the elevated temperature in the presence of condensed water vapor on portions of the sensor. Thermal shock resulting from heating the sensor in the presence of water condensate may cause cracking of elements in the sensor.
One solution to the problem of thermal shock in the presence of water condensate has been to delay the heating of the sensor for a predetermined length of time until the vehicle engine heats up and the engine temperature boils off the condensate. Unfortunately, it is difficult to determine the appropriate length of time delay before applying heater power. If the delay is too short, water condensate may still be present and element cracking may occur. If the delay is too long, although the problem of thermal shock can be avoided, the usefulness of the sensor is delayed because the sensor is not fully effective unless heated to the elevated temperature. The problem of thermal shock is most pronounced at vehicle start up before the exhaust system is heated to its full operating temperature. Unfortunately, the need for a properly functioning oxygen sensor is also most pronounced at start up and shortly thereafter when exhaust emissions are most problematic because the engine is cold. It is at this time that it is most important to be able to analyze the vehicle exhaust and to optimize the vehicle operating conditions based on that analysis.
Another solution to the problem of thermal shock has been to model the temperature of the skin of the exhaust pipe in the vicinity of the oxygen sensor. When this temperature exceeds the dew point, the heater power can be increased. However, it is difficult to make the models sufficiently accurate. For example, low points in the exhaust pipe well ahead of the sensor can collect or accumulate liquid water and these might not be accounted for in the model.
Accordingly, it is desirable to provide a method for calibrating and for controlling the heating of an oxygen sensor mounted in a motor vehicle to both optimize the performance of that sensor and to protect the sensor from thermal shock. In addition, it is desirable to provide an improved oxygen sensor and a system incorporating such an oxygen sensor in a motor vehicle. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.